Since their first discovery by Iijima in 1991, single-walled carbon nanotubes (SWNTs) have attracted much attention due to their unique structural and electronic properties. Results on such materials have added to the body of knowledge gathering on spherical fullerenes, such as C60, C70, and rarer, higher fullerenes such as C76, C78, C82 and C84. Several processes for large-scale synthesis/manufacture of SWNTs have also been developed by various research groups around the globe, and nearly all of them use transition metal catalysts to catalyze the process. Among of these catalysts, iron, cobalt, and nickel are quite common, with molybdenum typically used as a promoter. For example the high-pressure carbon monoxide (CO) (HiPCO) used by Carbon Nanotechnologies Inc (CNI) uses Fe as the catalyst. Typically, the carbon nanotubes (CNTs) produced by these methods usually contain both residual catalytic metal and nontubular carbonaceous species such as amorphous carbon, microcrystalline carbon soot or graphitic carbon. Fullerene synthesis, for example from graphite rods via arc discharge evaporation, may also result in the presence of carbon and metal impurities. These impurities are difficult to control, and are largely buried in the powdery support material or enclosed by the catalytic metal in the form of so-called onionated structure, and are therefore difficult to remove. Despite sustained efforts at conventional purification protocols in prior art efforts, there still exist significant quantities of the above mentioned impurities in all SWNT preparations using currently employed synthetic techniques. This necessitates the development of efficient, cost-effective and environmentally-friendly purification procedures, without which this unique material will have limited applications.
The conventional method used for purification of raw CNTs is by an acid treatment process to remove impurities which is time-consuming, and generally requires several days of stirring or refluxing or several hours of sonication. Moreover, all these methods lead to chemical-functionalization of the sidewalls and tips of the nanotubes resulting in modification or degradation of physical properties and defect-induced structural damage of the nanotubes. So, rapid and efficient purification methods are needed to dissolve the metal particles and concomitantly oxidize the amorphous carbon structures without damaging the nanotubes. Nanotube clean up is a key technology for successful commercialization of these materials. Current methods have limitations because they do not address all the complexities involved in the process. No off-the-shelf technology is directly applicable either. The important feature of a suitable technique would to be able to selectively remove metals, contaminants and residual non-tubular carbons while leaving the nanotubes undamaged.
This invention describes in some embodiments a rapid, efficient and environmentally friendly microwave-induced process for the removal of residual transition metal catalysts and non-desirable carbonaceous materials from nanocarbon preparations.